Besides Sea Bass and JPM, when was the last time we've seen a driver come from IndyCar/CART/Champ Car and get a ride in F1? Better yet, when was the last time a driver come from IndyCar/CART/Champ Car get in a competitive ride in F1?
In terms of American drivers, Michael Andretti was the last in 1993, with a top notch McLaren ride.
In the United States NASCAR has been king for some time now. After the American openwheel racing split in 1996 one after another top American openwheel drivers moved to NASCAR early in their career because openwheel was in shambles and would never attract the intrest of F1 teams, so they moved to NASCAR for either money or like Tony Stewart they wanted to be in the top motor racing series in America.
In America the local track is usualy a short oval or drag strip, that's why road course aces are few in the states. In Brazil and other places they are crazy over road racing and have a million go-kart tracks and that's why they produce so many good F1 drivers.
It's all about numbers and percentages.
An average kid in some other country grows up watching Formula 1 and they go and try to start their career in go-karts aiming for F1.
In the US the average kid grows up watching NASCAR, and he races the local oval track looking for a career in NASCAR.
NASCAR drivers who started out in openwheel like Jeff Gordon, Tony Stewart, AJ Almendinger and others have shown they have the talent and experience to compete for a F1 ride but moved to NASCAR for reason stated above.
I don't think a NASCAR driver can make the transition to F1 not because they aren't talented enough, but because the learning curve would be too high and F1 is not like NASCAR where you can have a JPM crash cars and run in the back of the pack for a couple of seasons before you put the pressure on him.
They expect results right away, even in your initial tests, and I doubt someone who has been racing heavy cars on oval for most of their life can make the transition fast enough to get a ride, let alone a top ride when there are thousands in line with a ton of road course and openwheel experience.
Once Indycars get their act together and the league starts making money and they can pay the top young American talent enough money to keep them away from NASCAR you will eventually see an American driver come from Indycars into F1. F1 will be more willing to take him then a NASCAR driver because of his openwheel experience on roadcourses.
It may take a long time because of what I mentioned in this post, how many kids do you know in America want to race F1, and how many can even find the opportunity when openwheel racing is in such dissaray.
Of the current Indycar drivers only Marco Andretti fits the bill of a probable American F1 racer because of his age and experience, but I don't think he is good enough. There are only 20 seats available in F1, and into get into one you either have to be a poster boy/prop for your country like Scott Speed or you have to be better then the thousand openwheel/sportscar/touring car etc drivers in the world looking for a ride in F1.
Is Marco Andretti better then those other drivers? I don't think so. So the wait for a good American driver who can make it in F1 goes on, and it may take forever depending on how fast Indycars rise back to prominence.